Lubricant composition for limiting friction

ABSTRACT

The present application relates to a lubricant composition comprising, in relation to the total weight of the lubricant composition at least one base oil; 0.005 to 10% by weight of at least one polymeric organic friction modifier; and 0.005 to 10% by weight of at least one ester said polymeric organic friction modifier is a compound of formula (I): 
       R 1 .[(AO) n .-AO—R 2 ] m    (I)
 
     wherein R 1  is a residue of a group having at least m hydrogen atoms, m being greater than 2;
     AO is an alkylene oxide residue;   n is between 0 and 100;   R 2  is a hydrogen atom or a C—(O)—R 3  group with R 3  being a residue.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase Application under 35U.S.C. § 371 of International Patent Application No. PCT/EP2020/085748filed Dec. 11, 2020, which claims priority of French Patent ApplicationNo. 19 14410 filed Dec. 13, 2019. The entire contents of which arehereby incorporated by reference.

FIELD OF THE INVENTION

The present application relates to new lubricant compositions, inparticular for reducing friction between mechanical parts, preferablybetween two parts of an engine, such as a vehicle engine. For example,the lubricant compositions according to the invention can be used tolubricate an internal combustion engine, in particular a vehicle engine,in particular a motor vehicle engine.

BACKGROUND

The purpose of lubricants is to reduce friction and wear of mechanicalparts, especially in vehicle engines, and more particularly in motorvehicles.

To reduce these friction phenomena, it is known to incorporate frictionmodifiers in lubricants.

Among friction modifiers, organomolybdenum compounds represent a familyof compounds whose friction-reducing properties have been widelydescribed. However, it is known to the person skilled in the art thatthe use of organomolybdenum compounds, in particular organomolybdenumcompounds comprising a dithiocarbamate group, can worsen the wearing ofmechanical parts. Other solutions were then proposed to reduce frictionbetween two mechanical parts.

Among these alternatives, polymeric organic friction modifiers arecurrently sometimes used.

For example, WO2011/116049 describes one type of polymeric organicfriction modifier of interest.

This type of polymeric friction modifier makes it possible to achievecoefficients of friction between mechanical parts that are sometimes toohigh for the applications envisaged.

There is therefore a particular interest in the provision of lubricantcompositions to reduce friction between mechanical parts.

SUMMARY

One objective of the present application is to provide lubricantcompositions for reducing friction between mechanical parts.

Further objectives will become apparent from the following descriptionof the invention.

DETAILED DESCRIPTION

These objectives are fulfilled by the present invention which provides alubricant composition comprising, based on the total weight of thelubricant composition:

-   -   at least one base oil;    -   0.005 to 10% by weight of at least one polymeric organic        friction modifier; and    -   0.005 to 10% by weight of at least one ester which is a product        of the esterification reaction between a saturated or        unsaturated, linear, cyclic or branched monohydric alcohol        having 1 to 10 carbon atoms and a carboxylic polyacid or between        a linear, cyclic or branched polyol and a saturated or        unsaturated, linear, cyclic or branched monocarboxylic acid        having between 1 and 10 carbon atoms,    -   said polymeric organic friction modifier is a compound of        formula (I):

R¹.[(AO)_(n).-AO—R²]_(m)   (I)

-   -   wherein R¹ is a residue of a group having at least m hydrogen        atoms, m being greater than 2;    -   AO is an alkylene oxide residue;    -   n is between 0 and 100;    -   R² is a hydrogen atom or a C—(O)—R³ group with R³ being a        residue selected from the list consisting of a polyhydroxyalkyl        carboxylic acid residue, a polyhydroxyalkenyl carboxylic acid        residue, a hydroxyalkyl carboxylic acid residue, a        hydroxyalkenyl carboxylic acid residue, a hydroxyalkyl        carboxylic acid oligomer residue and a hydroxyalkenyl carboxylic        acid oligomer residue; and    -   in which on average at least two R² groups are acyls.

More particularly, the inventors have surprisingly discovered that thecombination of a polymeric organic friction modifier of the above typeand an ester preferably selected from glycerol esters, citric acidesters, tartaric acid esters and mixtures thereof significantly improvesthe friction coefficient between mechanical parts.

Indeed, it has been discovered by the inventors that the ester chosenmore particularly among glycerol esters, citric acid esters, tartaricacid esters and their mixture allows to boost surprisingly the effect ofthe polymeric organic friction modifier.

According to another preferred alternative, the polymeric organicfriction modifier has a weight average molecular weight of from 3,000 to8,000 Daltons. The weight average molecular weight can be measured bysteric exclusion chromatography.

Application WO2011/116049 describes, for example, polymeric organicfriction modifiers.

The polymeric organic friction modifier is built around a central groupR¹.

The central group R¹ is the residue of a compound containing at least mhydrogen atoms, obtained after removal of said m hydrogen atoms.

Preferably, the m hydrogen atoms are hydrogen atoms of groups selectedfrom amino groups and hydroxyl groups, and are advantageously hydrogenatoms of hydroxyl groups.

Preferably, the central group R¹ is the residue of a substitutedhydrocarbyl group, in particular a C3 to C30 substituted hydrocarbylcompound.

Preferably, the central group R¹ is a residue of a compound containingat least m hydrogen atoms, obtained after removal of m hydrogen atoms,said compound being selected from:

-   -   glycerol and polyglycerols, in particular diglycerol and        triglycerol, partial esters of glycerol and polyglycerol,        triglycerides containing at least two hydroxyl groups, for        example castor oil;    -   triols, polyols, e.g. trimethylolethane, trimethylolpropane and        pentaerythritol, partial esters of polyols;    -   sugars, in particular non-reducing sugars, e.g. sorbitol,        mannitol and lactitol, etherified sugar derivatives, e.g.        sorbitan (cyclic dehydroether of sorbitol), partial alkyl        acetals of sugars, e.g. methyl glucose, alkyl saccharides, alkyl        polysaccharides, oligomers and polymers of sugars, e.g. dextrins        partially esterified sugar derivatives, e.g. fatty acid esters,        preferably selected from lauric acid, palmitic acid, oleic acid,        stearic acid and behenic acid, sorbitan esters, sorbitol esters,        sucrose esters, aminosaccharides, e.g. N-alkylglucamines and the        corresponding N-alkyl-N-alkenoyl glucamides;    -   polyhydroxy carboxylic acids, in particular citric acid and        tartaric acid;    -   amines, including di- and polyfunctional amines, in particular        alkylamines, including alkyl diamines such as ethylene diamine        (1,2-diaminoethane);    -   amino alcohols, in particular ethanolamines, 2-aminoethanol,        di-ethanolamine and triethanolamine;    -   carboxylic acid amides, for example urea, malonamide and        succinamide; and    -   amido-carboxylic acids, for example succinamic acid.

Preferably, the central group R¹ is a residue of a compound containingat least m hydrogen atoms, obtained after removal of m hydrogen atoms,said compound having at least 3, preferably 4 to 10, in particular 5 to8, advantageously 6 groups selected from amino groups and hydroxylgroups.

Preferably, the central group R¹ is a residue of a compound comprisingat least 3, preferably 4 to 10, in particular 5 to 8, advantageously 6hydroxyl groups.

Even more preferably, the central group R¹ comprises a C4 to C7 linearchain, more preferably C6.

The hydroxyl or amino groups are preferably directly linked to thecarbon atoms of the linear chain of the central group R¹.

Advantageously, the central group R¹ is a residue of a compound selectedfrom open chain tetratol, open chain pentitol, open chain hexitol andopen chain heptitol, or an anhydro compound derived from a compoundselected from tetratol, pentitol, hexitol and heptitol, for example ananhydro cycloether group derived from a compound selected from tetratol,pentitol, hexitol and heptitol.

In a particularly preferred embodiment, the central group R¹ is aresidue of a sugar, more preferably of a monosaccharide, preferablyselected from glucose, fructose and sorbitol, of a disaccharide,preferably selected from maltose, palitose, lactitol and lactose, or ofan oligosaccharide with a degree of polymerisation higher than 2.

Advantageously, the central group R¹ is the residue of a monosaccharide,preferably selected from glucose, fructose and sorbitol, and inparticular a sorbitol residue.

The central group R¹ is preferably in open chain form. However, thecentral group R¹ may also comprise an internal cyclic ether functionwhen the central group R¹ synthesis pathway exposes it to relativelyhigh temperatures or other conditions that favour such cyclisation.

The index m is a measure of the functionality of the central group R¹.

The index m is preferably greater than 3, preferably greater than orequal to 4 and less than or equal to 10, in particular greater than orequal to 5 and less than or equal to 8, advantageously greater than orequal to 5 and less than or equal to 6.

The index m can be a whole number or a decimal.

The R² groups are the terminal groups of the (poly)alkylene oxide chainsof the polymeric organic friction modifier of formula (I).

R² is a hydrogen atom or a C—(O)—R³ group with R³ being a residue of apolyhydroxyalkyl carboxylic acid, a residue of a polyhydroxyalkenylcarboxylic acid, a residue of a hydroxyalkyl carboxylic acid, a residueof a hydroxyalkenyl carboxylic acid, a residue of an oligomer of ahydroxyalkyl carboxylic acid and/or a residue of an oligomer of apolyhydroxyalkenyl carboxylic acid.

Hydroxyalkyl carboxylic acid and hydroxyalkenyl carboxylic acid have theformula HO—X—COOH, wherein X is a divalent saturated or unsaturated,preferably saturated, aliphatic radical containing at least 8 and atmost 20 carbon atoms, typically from 11 to 17 carbon atoms, and in whichthere are at least 4 carbon atoms between the hydroxyl group and thecarboxylic acid group.

Preferably, the hydroxyalkyl carboxylic acid is 12-hydroxystearic acid.

In practice, hydroxyalkyl carboxylic acids are commercially available asmixtures of the hydroxylic acid and the corresponding unsubstitutedfatty acid. For example, 12-hydroxystearic acid is typically made byhydrogenating castor oil fatty acids comprising the unsaturated C18hydroxyl acid and the unsubstituted fatty acids (oleic and linoleicacids) which, on hydrogenation, give a mixture of 12-hydroxystearic acidand stearic acid.

Commercially available 12-hydroxystearic acid typically contains about5-8% unsubstituted stearic acid.

Polyhydroxyalkyl carboxylic acid and polyhydroxyalkenyl carboxylic acidare manufactured by polymerisation of hydroxyalkyl carboxylic acid orhydroxyalkenyl carboxylic acid, respectively. Hydroxyalkyl carboxylicacid and hydroxyalkenyl carboxylic acid are as defined above.

The presence of the corresponding unsubstituted fatty acid incommercially available hydroxyalkyl carboxylic acids acts as aterminating agent and thus limits the chain length of the polymer.Preferably, the number of monomer units in the polyhydroxyalkylcarboxylic acid and in the polyhydroxyalkenyl carboxylic acid is onaverage from 2 to 10, preferably from 4 to 8 and advantageously about 7.

The molecular weight of the polyhydroxyalkyl carboxylic acid andpolyhydroxyalkenyl carboxylic acid is typically 600 to 3000 g/mol, inparticular 900 to 2700 g/mol, more particularly 1500 to 2400 g/mol andadvantageously about 2100 g/mol.

Polyhydroxyalkyl carboxylic acid and polyhydroxyalkenyl carboxylic acidare characterised by a residual acid number of less than 50 mg KOH/g,preferably between 30 and 35 mg KOH/g.

Preferably, the hydroxyl value of the polyhydroxyalkyl carboxylic acidand the polyhydroxyalkenyl carboxylic acid is less than or equal to 40mg KOH/g, advantageously between 20 and 30 mg KOH/g.

The hydroxyalkyl carboxylic acid oligomer and polyhydroxyalkenylcarboxylic acid oligomer differ from polyhydroxyalkyl carboxylic acidand polyhydroxyalkenyl carboxylic acid in that the terminus is not thecorresponding unsubstituted fatty acid. Desirably, the hydroxyalkylcarboxylic acid oligomer and the polyhydroxyalkenyl carboxylic acidoligomer are dimers of hydroxylalkyl carboxylic acid and hydroxyalkenylcarboxylic acid, respectively.

The alkylene oxide residue AO is a group of the formula —(C_(r)H_(2r)O),where r is 2, 3 or 4, preferably 2 or 3, i.e. an ethylene oxide residue(—C₂H₄O—) or a propylene oxide residue (—C₃H₆O—). AO can representdifferent groups along the alkylene oxide chain (AO)_(n).

Preferably, (AO)_(n) is a homopolymeric chain of the formula(—C₂H₄O—)_(n), n being between 1 and 100.

Alternatively, (AO)_(n) is a homopolymeric chain of porpylene oxidegroup of the formula (—C₃H₆O—)_(n), where n is between 1 and 100.

Alternatively, (AO)_(n) is a block or random copolymer chain containingboth ethylene oxide (—C₂H₄O—) and propylene oxide (—C₃H₆O—) residues.According to this embodiment, the molar proportion of ethylene oxideunits (—C₂H₄O—) in the copolymer chain is at least 50%, preferably atleast 70%.

The parameter n represents the number of alkylene oxide residues in the(poly)alkylene oxide (AO)_(n)chains. Preferably, n is between 2 and 50,preferably between 3 and 20, advantageously between 5 and 10. The totalof the indices n (i.e. n x m) is preferably between 10 and 300,preferably between 20 and 100, in particular between 5 and 70,advantageously between 30 and 50. The value of the index n is an averagevalue, which includes the statistical variation of the length of thechain.

When the number of acyl residues —C—(O)—R³ in the polymeric organicfriction modifier of formula (I) is significantly less than m, thedistribution of these groups depends on the nature of the central groupR¹.

When the central group R¹ is derived from pentaerythritol, thealkoxylation of pentaerythritol is evenly distributed over the fouravailable sites to remove one hydrogen, and the distribution of acylgroups is close to the expected random distribution.

When the central group R¹ is derived from compounds where the m hydrogenatoms are not equivalent, e.g. sorbitol, the alkoxylation will giveunequal lengths of (poly)alkylene oxide chains. The introduction byesterification of —C—(O)—R³ residues onto the shorter (poly)alkyleneoxide chains can be relatively difficult due to the strong stericeffects exerted by the longer (poly)alkylene oxide chains. Theintroduction by esterification of —C—(O)—R³ acyl residues in this casetakes place preferentially in the longer (poly)alkylene oxide chains.

The polymeric organic friction modifier of formula (I) is prepared fromthe compound containing at least m hydrogen atoms.

The first step in the preparation of the polymeric organic frictionmodifier of formula (I) is an alkoxylation of the groups containing theat least m hydrogen atoms. Alkoxylation is carried out by techniqueswell known to the skilled person, for example by reacting the compoundcontaining at least m hydrogen atoms with the required amounts ofalkylene oxide, for example ethylene oxide and/or propylene oxide.

The second step in the preparation of the polymeric organic frictionmodifier of formula (I) is to react the alkoxylated species obtainedfrom the first step with a polyhydroxyalkyl carboxylic acid and/or apolyhydroxyalkenyl carboxylic acid and/or a hydroxyalkyl carboxylic acidand/or a hydroxyalkenyl carboxylic acid under standard catalysedesterification conditions at temperatures up to 250° C.

The lubricant composition according to the invention comprises from0.005 to 10 wt %, preferably from 0.05 to 5 wt %, more preferably from0.1 to 3 wt %, more preferably from 0.2 to 2 wt %, of polymeric organicfriction modifier(s) as defined above, based on the total weight of thelubricant composition

The lubricant composition according to the invention comprises from0.005 to 10% by weight, preferably from 0.05 to 5% by weight, preferablyfrom 0.1 to 3% by weight, more preferably from 0.2 to 2% by weight, ofat least one ester selected from glycerol esters, citric acid esters,tartaric acid esters, and mixtures thereof, based on the total weight ofthe lubricant composition.

The ester used according to the invention may be a mono-, di- ortri-ester. It can be a mixture of mono-, di- and/or tri-esters.Preferably, the ester used according to the invention comprises at leastone triester.

Preferably, the ester is selected from glycerol esters, citric acidesters and mixtures thereof.

According to one embodiment of the invention, the glycerol ester is anester of glycerol and a carboxylic acid having from 1 to 10 carbonatoms, preferably from 2 to 8 carbon atoms. Preferably, the carboxylicacid is a monocarboxylic acid. In one embodiment of the invention, theglycerol ester is selected from glycerol heptanoates and mixturesthereof.

The carboxylic acids used to prepare the glycerol ester are saturated orunsaturated, linear, cyclic or branched carboxylic acids, optionallysubstituted with hydroxyl and/or epoxy groups.

Preferably, the carboxylic acid used to prepare the glycerol ester islinear and saturated and has a hydrocarbon chain consisting of carbonand hydrogen atoms. In other words, according to a particularembodiment, the carboxylic acid used to prepare the glycerol ester doesnot comprise any heteroatoms other than those of the acid function.

In one embodiment, the glycerol ester is obtained from raw materials ofrenewable origin. The carboxylic acids that can be used to form theglycerol ester are, for example, carboxylic acids derived from vegetableoils, fats, of animal or vegetable origin, such as butyric acid, valericacid, caproic acid, heptylic acid, caprylic acid, pelargonic acid,capric acid, crotonic acid, iso-crotonic acid, sorbic acid, isovalericacid, taken alone or mixed. In another embodiment, the glycerol ester isobtained from raw materials of fossil origin. These are known assynthetic carboxylic acids. Synthetic carboxylic acids such as butanoicacid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid,nonanoic acid, decanoic acid, taken alone or mixed, may also be used.

The glycerol esters used in the invention can be obtained by methodswell known to the skilled person, for example by reacting carboxylicacids with glycerol. These chemical reactions, which are well known tothe skilled person, can take place with or without a catalyst, with orwithout a solvent.

According to one embodiment, the glycerol ester used in the lubricantcomposition according to the invention is glycerol triheptanoate.

According to one embodiment, the tartaric acid ester is an ester oftartaric acid and an alcohol having from 1 to 10 carbon atoms,preferably from 2 to 8 carbon atoms. Preferably, the alcohol used toprepare the tartaric acid ester is a monoalcohol.

In one embodiment of the invention, the tartaric acid ester is selectedfrom tartaric acid triesters.

According to one embodiment, the citric acid ester is an ester of citricacid and an alcohol having 1 to 10 carbon atoms, preferably 2 to 8carbon atoms. Preferably, the alcohol used to prepare the citric acidester is a monoalcohol.

In one embodiment of the invention, the citric acid ester is selectedfrom citric acid triesters.

The alcohols used to prepare the citric acid ester or tartaric acidester are saturated or unsaturated, linear, cyclic or branched alcohols,optionally substituted by acid and/or epoxy groups.

Preferably, the alcohol used to prepare the citric acid ester ortartaric acid ester is linear and saturated and has a hydrocarbon chainconsisting of carbon and hydrogen atoms. In other words, according to aparticular embodiment, the alcohol used to prepare the citric acid esteror tartaric acid ester does not comprise any heteroatoms other thanthose of the hydroxyl function.

The citric acid esters or tartaric acid esters used in the invention canbe obtained by methods well known to the person skilled in the art, forexample by reacting citric acid or tartaric acid with one or morealcohols. These chemical reactions, which are well known to the skilledperson, can take place with or without a catalyst, with or without asolvent.

In one embodiment, the citric acid ester is selected fromtriethylcitrate, tributylcitrate and mixtures thereof.

According to an embodiment of the invention, the ester of the lubricantcomposition is selected from:

-   -   a triester of glycerol and of a monocarboxylic acid having from        1 to 10 carbon atoms, preferably from 2 to 8 carbon atoms; and    -   a triester of citric acid and of a monohydric alcohol having 1        to 10 carbon atoms, preferably 2 to 8 carbon atoms; and    -   mixtures thereof.

According to one embodiment of the invention, the ester of the lubricantcomposition is selected from glycerol triheptanoate, triethylcitrate,tributylcitrate and mixtures thereof.

The lubricant composition according to the invention comprises one ormore base oils, preferably in an amount of at least 50% by weight, morepreferably at least 60% by weight or even at least 70% by weight, basedon the total weight of the lubricant composition.

The base oil(s) may be selected from the mineral, synthetic or natural,animal or vegetable lubricating base oils known to the skilled person.

The base oils used in the lubricant compositions according to theinvention may be oils of mineral or synthetic origin belonging to groupsI to V according to the classes defined in the API classification (ortheir equivalents according to the ATIEL classification) (table 1) ormixtures thereof.

TABLE 1 Saturated Viscosity index content Sulphur content (VI) GroupingI  <90% >0.03% 80 ≤ VI < 120 Mineral oils Grouping II ≥90% ≤0.03% 80 ≤VI < 120 Hydrocracked oils Grouping III ≥90% ≤0.03% ≥120 Hydrocracked orhydro- isomerised oils Grouping IV Polyalphaolefins (PAO) Grouping VEsters, PAGs and other bases not included in Groupings I to IV

Mineral base oils according to the invention include all types of baseoils obtained by atmospheric and vacuum distillation of crude oil,followed by refining operations such as solvent extraction,de-alkalization, solvent dewaxing, hydrotreating, hydrocracking,hydroisomerization and hydrofining.

Mixtures of synthetic and mineral oils can also be used.

There is generally no limitation on the use of different lubricatingbases to make the lubricant compositions according to the invention,except that they must have properties, in particular viscosity,viscosity index, sulphur content, resistance to oxidation, suitable fortheir use.

The base oils of the lubricant compositions according to the inventionmay also be chosen from synthetic oils, such as certain esters ofcarboxylic acids and alcohols, and from polyalphaolefins.Polyalphaolefins used as base oils are for example obtained frommonomers with 4 to 32 carbon atoms, for example from octene or decene,and whose viscosity at 100° C. is between 1.5 and 15 mm².s⁻¹ accordingto ASTM D445. Their average molecular weight is generally between 250and 3,000 according to ASTM D5296.

According to one particular embodiment, the lubricant compositionaccording to the invention comprises from 60% to 99.5% by weight of baseoils, preferably from 70% to 99.5 by weight of base oils, relative tothe total weight of the composition.

Many additional additives can be used for this lubricant compositionaccording to the invention.

Preferred additional additives for the lubricant composition accordingto the invention are selected from detergent additives, anti-wearadditives other than phospho-sulphur additives, friction modifyingadditives other than the polymeric organic friction modifiers definedabove, extreme pressure additives, dispersants, pour point depressants,anti-foaming agents, thickeners and mixtures thereof.

Preferably, the lubricant composition according to the inventioncomprises, based on the total weight of lubricant composition:

-   -   at least 50% by weight, preferably at least 60% by weight, more        preferably 70% by weight, of one or more base oils;    -   from 0.005 à 10% by weight, preferably 0.05 to 5% by weight,        preferably from 0.1 to 3% by weight, more preferably from 0.2 to        2% by weight, of one or more polymeric organic friction        modifiers;    -   from 0.005 to 10% by weight, preferably from 0.05 to 5% by        weight, preferably from 0.1 to 3% by weight, more preferably        from 0.2 to 2% by weight, of one or more esters selected from        glycerol esters, citric acid esters, tartaric acid esters and        mixtures thereof;    -   optionally from 0.005 to 30% by weight, preferably from 0.1 to        25% by weight, more preferably from 1 to 20% by weight, of one        or more functional additives other than polymeric organic        friction modifiers and glycerol esters, citric acid esters and        tartaric acid esters preferably selected from detergent        additives, anti-wear additives other than phospho-sulphur        additives, friction modifying additives, extreme pressure        additives, dispersants, pour point depressants, anti-foaming        agents, thickeners and mixtures thereof.

Amine phosphates are anti-wear additives which can be used in thelubricant composition according to the invention. However, thephosphorus provided by these additives can act as a poison forautomotive catalytic systems as these additives are ash generators.These effects can be minimised by partially substituting aminephosphates with non-phosphorous additives, such as polysulphides,especially sulphur-containing olefins.

Advantageously, the lubricant composition according to the invention maycomprise from 0.01 to 6 wt. %, preferably from 0.05 to 4 wt. %, morepreferably from 0.1 to 2 wt. % based on the total weight of lubricantcomposition, anti-wear additives and extreme pressure additives.

Advantageously, the lubricant composition according to the invention maycomprise at least one additional friction modifier additive differentfrom the polymeric organic friction modifiers defined above. Theadditional friction modifier additive may be selected from a compoundproviding metallic elements and an ash-free compound. Among thecompounds providing metallic elements, we can mention transition metalcomplexes such as Sb, Sn, Fe, Cu, Zn, Mo whose ligands can behydrocarbon compounds comprising oxygen, nitrogen, sulphur or phosphorusatoms.

Advantageously, the lubricant composition according to the invention maycomprise at least one antioxidant additive.

The antioxidant additive generally delays the degradation of thelubricant composition in service. This degradation may result in theformation of deposits, the presence of sludge or an increase in theviscosity of the lubricant composition.

Antioxidant additives act as radical inhibitors or hydroperoxidedestroyers. Commonly used antioxidant additives include phenolicantioxidant additives, amine antioxidant additives and phosphosulphurantioxidant additives. Some of these antioxidant additives, e.g.phosphosulphur antioxidant additives, can be ash-forming. Phenolicantioxidant additives can be ash-free or in the form of neutral or basicmetal salts. The antioxidant additives may in particular be selectedfrom sterically hindered phenols, sterically hindered phenol esters andsterically hindered phenols comprising a thioether bridge,diphenylamines, diphenylamines substituted by at least one C₁-C₁₂, alkylgroup, N,N′-dialkyl-aryl-diamines and mixtures thereof.

Preferably according to the invention, the sterically hindered phenolsare chosen from compounds comprising a phenol group in which at leastone vicinal carbon of the carbon bearing the alcohol function issubstituted by at least one C₁-C₁₀, alkyl group, preferably a C₁-C₆,alkyl group, preferably a C₄, alkyl group, preferably by the ter-butylgroup.

Amino compounds are another class of antioxidant additives that can beused, possibly in combination with phenolic antioxidant additives.Examples of amino compounds are aromatic amines, for example aromaticamines of the formula NR⁷R⁸R⁹ where R⁷ represents an aliphatic group oran aromatic group, optionally substituted, R⁸ represents an aromaticgroup, optionally substituted, R⁹ represents a hydrogen atom, an alkylgroup, an aryl group or a group of formula R¹⁰S(O)_(z)R¹¹ in which R¹⁹represents an alkylene group or an alkenylene group, R¹¹ represents analkyl group, an alkenyl group or an aryl group and z represents 0, 1 or2.

Sulphurised alkyl phenols or their alkali and alkaline earth metal saltscan also be used as antioxidant additives.

Another class of antioxidant additives are copper compounds, e.g. copperthio- or dithio-phosphates, copper salts of carboxylic acids,dithiocarbamates, sulphonates, phenates, copper acetylacetonates. CopperI and II salts, succinic acid or anhydride salts can also be used.

The lubricant composition according to the invention may contain anytype of antioxidant additives known to the person skilled in the art.

Advantageously, the lubricant composition comprises at least oneash-free antioxidant additive.

Equally advantageously, the lubricant composition according to theinvention comprises from 0.5 to 2% by weight, based on the total mass ofthe composition, of at least one antioxidant additive.

The lubricant composition according to the invention may also compriseat least one detergent additive.

Detergent additives generally reduce the formation of deposits on thesurface of metal parts by dissolving oxidation and combustionby-products.

The detergent additives used in the lubricant composition according tothe invention are generally known to the person skilled in the art.Detergent additives can be anionic compounds comprising a longlipophilic hydrocarbon chain and a hydrophilic head. The associatedcation may be a metal cation of an alkali or alkaline earth metal.

The detergent additives are preferably selected from alkali or alkalineearth metals of carboxylic acids, sulphonates, salicylates,naphthenates, and phenate salts. The alkali and alkaline earth metalsare preferably calcium, magnesium, sodium or barium.

These metal salts generally contain the metal in a stoichiometric amountor in excess, i.e. in an amount greater than the stoichiometric amount.These are overbased detergent additives; the excess metal giving theoverbased character to the detergent is generally in the form ofoil-insoluble metal salts, e.g. carbonate, hydroxide, oxalate, acetate,glutamate, preferably carbonate.

Advantageously, the lubricant composition according to the invention maycomprise from 2% to 4% by weight of detergent additive relative to thetotal weight of the lubricant composition.

Equally advantageously, the lubricant composition according to theinvention may also comprise at least one pour point depressant additive.

By slowing down the formation of paraffin crystals, pour pointdepressant additives generally improve the cold behaviour of thelubricant composition according to the invention.

Examples of pour point depressant additives are alkyl polymethacrylates,polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes,alkylated polystyrenes.

Advantageously, the lubricant composition according to the invention mayalso comprise at least one dispersing agent.

The dispersing agent may be selected from Mannich bases, succinimidesand derivatives thereof.

Advantageously, the lubricant composition according to the invention maycomprise from 0.2% to 10% by weight of dispersant(s) based on the totalweight of the lubricant composition.

The lubricant composition of the present invention may also comprise atleast one additional polymer that can improve the viscosity index.Examples of additional viscosity index improving polymers are polymericesters, hydrogenated or non-hydrogenated homopolymers or copolymers ofstyrene, butadiene and isoprene, polymethacrylates (PMA).

The present invention also relates to the use of the lubricantcomposition as defined above for the lubrication of metal parts, inparticular for the lubrication of engines, in particular internalcombustion engines, for example vehicle engines.

Advantageously, the lubricant composition according to the inventionmakes it possible to reduce friction, in particular between twomechanical parts, for example two parts of an engine, in particular aninternal combustion engine, for example a vehicle engine.

Thus, the invention relates to the use of the lubricant compositionaccording to the invention to reduce the wear of mechanical parts, forexample parts of an engine, in particular a vehicle engine.

The present application also relates to a method of lubricatingmechanical parts, particularly in an engine, such as an internalcombustion engine, comprising at least one step of bringing a part intocontact with the lubricating composition according to the invention.

The present invention will now be described with the help ofnon-limiting examples.

EXAMPLE 1 Lubricant Compositions

The compositions in Table 2 (LC: lubricant composition according to theinvention; CC: comparative composition) were prepared by mixing at 60°C. the ester and/or polymeric friction modifier in a compositioncomprising base oil, viscosity index improver and additive package, togive the proportions shown in Table 2. The percentages shown are basedon 100% by weight of the lubricant composition including the esterand/or polymeric friction modifier.

TABLE 2 Table 2. Lubricant compositions according to the invention andcomparative ones LC6 CC2 Lubricating composition (% by weight) (% byweight) triethylcitrate 1 1 Organic friction modifier 0.5 — according tothe invention* Additive package 1** 14.2 13.8 Viscosity index improver(olefin 6.0 6.1 copolymer) Group III base oil 78.3 79.1 *An organicfriction modifier which is a compound of formula (I):R¹•[(AO)_(n•)-AO—R²]_(m) (I) wherein R¹ is a residue of a group havingat least m hydrogen atoms, m being greater than 2 AO is an alkyleneoxide residue n is between 0 and 100 R² is a hydrogen atom or a C—(O)—R³group with R³ being a residue selected from the list consisting of apolyhydroxyalkyl carboxylic acid residue, a polyhydroxyalkenylcarboxylic acid residue, a hydroxyalkyl carboxylic acid residue, ahydroxyalkenyl carboxylic acid residue, a hydroxyalkyl carboxylic acidoligomer residue and a hydroxyalkenyl carboxylic acid oligomer residue;and in which on average at least two R² groups are acyls. **comprisingdetergents, dispersants, antioxidants and anti-wear agents

EXAMPLE 2 Tribological Test Results

The tribological tests were carried out under the following conditions:

TABLE 3 Table 3. Tribological test conditions Slip-slip Filler 30N (0.96Gpa) Temperature 100° C. SRR 50% Entrainment speed 0.1 m/s Time (min) 515 30 60 120 Stribeck Filler 30N (0.96 Gpa) Temperature 100° C. SRR 50%Entrainment speed 3 to 0.007 m/s

The coefficient of friction of the lubricant compositions tested isdetermined at 100° C. using an MTM (Mini Traction Machine) device usinga 2 cm diameter hardened steel ball on a hardened steel plane.

The MTM device can be a PCS Instruments device. This device allows asteel ball and a steel plane to be moved relative to each other in orderto determine the coefficients of friction for a given lubricantcomposition while varying various properties such as speed, load, andtemperature.

The hardened steel plane is AISI 52100 with a mirror finish (Ra lessthan 0.01 μm) and the ball is also AISI 52100 made of hardened steel.

The applied load is 30 N (0.96 Gpa) and the rotation speed varies from0.007 m/s to 3 m/s.

Approximately 50 ml of the tested lubricant composition was introducedinto the device. The ball is engaged face to face with the plane, saidball and plane being independently actuated so as to create a mixedrolling/sliding contact.

The coefficient of friction is measured and recorded by means of a forcesensor.

The test is conducted for a duration of 121 minutes (alternating betweenslip-slip and Stribeck periods). The velocity is initially held constantat 0.1 m/s and at each interval defined in the table, the velocity isincreased from 3 to 0.007 m/s for one minute before returning to avelocity of 0.1 m/s at the end of said defined period.

The coefficient of friction is thus measured as a function of thedefined speed.

Table 4 gives the results for the compositions in Table 2, expressed interms of coefficient of friction versus slip speed.

TABLE 4 Speed of 0.01 m/s Speed of 0.1 m/s Coefficient of friction CC20.133 0.112 Coefficient of friction LC6 0.033 0.025

The results show that:

-   -   the ester has no significant effect on the coefficient of        friction when used alone, without a polymeric friction modifier.    -   there is a synergy between the ester defined herein and the        polymeric friction modifiers within the lubricant composition to        significantly decrease the coefficient of friction and therefore        to limit friction between mechanical parts.

1. A lubricant composition comprising, based on the total weight oflubricant composition: at least one base oil; 0.005 to 10% by weight ofat least one polymeric organic friction modifier; and 0.005 to 10% byweight of at least one ester which is a product of the esterificationreaction between a saturated or unsaturated, linear, cyclic or branchedmonohydric alcohol having 1 to 10 carbon atoms and a carboxylic polyacidor between a linear, cyclic or branched polyol and a saturated orunsaturated, linear, cyclic or branched monocarboxylic acid havingbetween 1 and 10 carbon atoms, said polymeric organic friction modifieris a compound of formula (I):R¹.[(AO)_(n).-AOR²]_(m)   (I) wherein R¹ is a residue of a group havingat least m hydrogen atoms, m being greater than 2; AO is an alkyleneoxide residue; n is between 0 and 100; R² is a hydrogen atom or aC—(O)—R³ group with R³ being a residue selected from the list consistingof a polyhydroxyalkyl carboxylic acid residue, a polyhydroxyalkenylcarboxylic acid residue, a hydroxyalkyl carboxylic acid residue, ahydroxyalkenyl carboxylic acid residue, a hydroxyalkyl carboxylic acidoligomer residue and a hydroxyalkenyl carboxylic acid oligomer residue;and in which on average at least two R² groups are acyls.
 2. Thecomposition according to claim 1, wherein the polymeric organic frictionmodifier has a weight average molecular weight of from 3,000 to 8,000Daltons.
 3. The composition according to claim 1, wherein the ester isselected from: a triester of glycerol and of a monocarboxylic acidhaving from 1 to 10 carbon atoms; and a triester of citric acid and of amonohydric alcohol having 1 to 10 carbon atoms; and mixtures thereof. 4.The composition according to claim 1, wherein the ester is selected fromglycerol triheptanoate, triethylcitrate, tributylcitrate and mixturesthereof.
 5. The composition according to claim 1, at least 50% by weightof one or more base oils; from 0.005 a 10% by weight of one or morepolymeric organic friction modifiers; from 0.005 to 10% by weight of oneor more esters selected from glycerol esters, citric acid esters,tartaric acid esters and mixtures thereof; optionally from 0.005 to 30%by weight of one or more functional additives other than polymericorganic friction modifiers and glycerol esters, citric acid esters andtartaric acid esters.
 6. A method for reducing friction between twomechanical parts comprising one step of bringing the two mechanicalparts into contact with the lubricating composition according toclaim
 1. 7. The method according to claim 6 for reducing the wear ofparts.
 8. A method for lubricating mechanical parts comprising at leastone step of bringing a part into contact with the lubricatingcomposition according to claim 1.